1. An IRC or MUD user
who is actually a program. On IRC, typically the robot provides some
useful service. Examples are NickServ, which tries to prevent random users
from adopting nicks already claimed by others, and
MsgServ, which allows one to send asynchronous messages to be delivered
when the recipient signs on. Also common are ‘annoybots’, such
as KissServ, which perform no useful function except to send cute messages
to other people. Service bots are less common on MUDs; but some others,
such as the ‘Julia’ bot active in 1990--91, have been
remarkably impressive Turing-test experiments, able to pass as human for as
long as ten or fifteen minutes of conversation.

2. An AI-controlled player in a computer game (especially a
first-person shooter such as Quake) which, unlike ordinary monsters,
operates like a human-controlled player, with access to a player's weapons
and abilities. An example can be found at http://www.telefragged.com/thefatal/.

3. Term used, though less commonly, for a web
spider. The file for controlling spider behavior on
your site is officially the “Robots Exclusion File” and its
URL is “http://<somehost>/robots.txt”)

Note that bots in all senses were ‘robots’ when the terms
first appeared in the early 1990s, but the shortened form is now
habitual.